Certain hot working processes for metals, such as rolling of steel and other processes conducted at relatively high temperatures, often leave an oxide layer or scale on the surface of the material worked. Removal of such scale can be accomplished by use of acids that dissolve the scale. The process of removing scale from metal by use of acids is commonly known as “pickling” of the metal.
The most commonly used pickling acid is hydrochloric acid, although certain applications may include use of sulfuric acid. Hydrochloric acid is more expensive than sulfuric acid, but its pickling effect on the metal is faster. A certain amount of metal is typically lost during the pickling process. Hydrochloric acid pickling processes are known to advantageously minimize the amount of metal loss.
Modem metal production and processing facilities require high rates of production, which makes use of hydrochloric acid as the pickling solution more desirable because if its capability to quickly de-scale metal. Nevertheless, the relatively higher cost of hydrochloric acid makes such de-scaling processes more expensive to operate, especially since the rate of loss of hydrochloric acid during these processes is also relatively high.
In a typical pickling process, hot or warm metal is passed in a continuous fashion through a bath or under a shower of the pickling liquid. The pickling liquid wets and washes the surface of the metal, thus dissolving and removing the scale that has accumulated thereon. In the case of a shower application of the liquid, which is often conducted at a high pressure to aid in the quick removal of the scale, a certain amount of the sprayed fluid disperses into the atmosphere in the form of an aerosol solution. The aerosol solution may contain about 40 parts per million (PPM) of gaseous or evaporated acid components, and about 1160 PPM of acid fog or mist. Because of environmental considerations, the gaseous and aerosol components of the pickling liquid are removed from the bath and filtered to remove therefrom or recover as much of the acid as possible for re-use into the system.
Pickling sludge is the waste product from pickling, and includes acidic rinse waters, metallic salts and waste acid. Spent pickle liquid is collected and processed to recover useful compounds, for example, Ferric Chloride (FeCl3), which can be resold to other industries for various uses, such as a coagulant in sewage treatment and drinking water production, an etchant for copper-based metals in printed circuit boards, and other applications.
Accordingly, there is a need to reduce waste of pickling liquids while also improving the yield and concentration of byproducts of the pickling process that have commercial value.